Poly-steel double wall tank

ABSTRACT

An improved double wall tank has two or more threaded fittings, with a steel outer tank and a thermoplastic inner tank. The inner tank is formed within the outer tank by rotational molding. The inner tank is bonded to the outer tank in an area of the fittings, but is otherwise separate and apart from the outer tank. Before carrying out the rotational molding process, bonding material is applied to the outer tank in the area of the fittings. The thermoplastic material is in powder form and includes mold release material. The thermoplastic material is inserted into the outer tank before rotational molding. The degree of shrinkage of the inner tank can be controlled by air pressure including an area between the fittings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved double wall tank having an outertank of steel and an inner tank of thermoplastic material. Moreparticularly, this invention has an outer tank with at least twofittings thereon and the inner tank is separate and apart from the outertank except that it is bonded to outer tank in the area of the fittings.

2. Description of the Prior Art

It is known to have outer steel tanks with inner tanks made fromthermoplastic material. The inner tank is formed by rotation moldingwithin the outer tank and separates from the outer tank when the tankcools (see U.S. Pat. No. 4,65,892). When the inner tank cools, thethermoplastic material shrinks. In tanks that have two or more fittings,as the inner tank shrinks, that portion of the inner tank wall locatedbetween the two fittings is placed under enormous stress. The stress cancause the inner tank to rupture or to fail prematurely at the cornerswhere the inner tank bends into the flanges for the fittings.

It is also known to line a tank with an inner layer of polymericmaterial where the polymeric material is bonded to an inner surface ofthe tank (see U.S. Pat. No. 5,358,682). An advantage of a double walltank where there is a space between the inner tank and the outer tank isthat a leak detection system can monitor the space between the twowalls.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an outer tank ofsteel having an inner tank of thermoplastic material where the outertank has at least two fittings and the bonding of the inner tank to theouter tank can be controlled so that the inner tank is bonded to theouter tank in the area of the fittings that is otherwise separate andapart from the outer tank.

A double wall tank has an outer tank of steel and an inner tank ofthermoplastic material. The outer tank has an inner surface having firstareas and a second area. One of the first areas is located adjacent toeach of at least two fittings that are separate from one another. Thesecond area is a remainder of the inner surface that is not the firstareas. The thermoplastic material is bonded to the outer tank in thefirst areas and separate from the outer tank in the second area.

A method of constructing a double wall tank having an outer tank ofsteel and an inner tank of thermoplastic material, said outer tankhaving an inner surface with first areas and a second area, one of saidfirst areas being located adjacent to each of at least two fittings thatare separate and apart from one another, said outer tank having anopening at each fitting, said second area being a remainder of saidinner surface that is not said first areas, said method comprisingapplying bonding material to said first areas, inserting saidthermoplastic material in powder form into said outer tank through anopening for a fitting, said powder including mold release material,inserting a plug into each opening, subjecting the outer tank to heatand rotating the outer tank to mold said inner tank within said outertank, cooling said tanks, resulting in an inner wall of saidthermoplastic material that is bonded to said first areas and separatedfrom said second area, removing each plug and cutting an opening throughsaid thermoplastic material at each opening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of prior art tank with two fittings andthe inner tank extending within the flanges;

FIG. 2 is a sectional side view of a tank having two fittings;

FIG. 3 is a sectional end view of the tank of FIG. 2;

FIG. 4 is a partial perspective sectional view of the tank of FIG. 2 atone of the fittings with a cover located between the fitting and a tankwall;

FIG. 5 is a partial sectional side view of a tank with a fitting affixeddirectly to a tank;

FIG. 6 is a partial sectional view of a tank where the section is takenthrough the fitting; and

FIG. 7 is a partial sectional sideview of a tank having a flangefitting.

DESCRIPTION OF A PREFERRED EMBODIMENT

In FIG. 1, there is shown a prior art double wall tank 2 where an outersteel tank 4 has an inner thermoplastic tank 6. The tank 2 has twoflanges 8 and two flange covers 10. One flange cover 10 covers each tankopening 12. The tank 2 is manufactured by rotational molding to mold theinner tank within the outer tank. The outer tank is heated during themolding process causing the inner tank to form on an inner surface 14 ofthe outer tank 4. There is a space 16 between the two tanks 4,6 whereleak detection equipment (not shown) can monitor for leaks. When the twotanks are cooled either as part of or after the molding process, theinner tank 6 shrinks relative to the outer tank 4, thereby creating thespace between the two tanks. The space and shrinkage may be controlledby the addition of air pressure into the inner tank during the coolingprocess. The shrinkage causes stress on the inner tank junctures 18,20,often resulting in stress cracking of the inner tank in an area 22between the junctures 18,20. When threaded fittings are required withthe prior art tank, they are supplied by attachment to each of theflange covers 10. Air pressure cannot alleviate the stress at junctures18, 20. Air pressure cannot be used to exert longitudinal pressure onthe inner tank in the area between the flanges because the inner tankcontains an opening at each flange.

In FIG. 2 there is shown a double wall tank 24 having an outer tank 26and an inner tank 28 with an interstice 30 located between the two tanks26, 28. The interstice can be used (usually by creating a vacuum) tomonitor the inner tank for leaks. The tank 24 has two fittings 32. Itcan be seen that the inner tank 28 is affixed to a cover 34 in two firstareas 36 adjacent to the fittings 32. The second area is a remainder ofan inner surface 38 of the outer tank 26.

In FIG. 3, there is shown a sectional end view of the tank 24. The samereference numerals are used in FIG. 3 as those used in FIG. 2 for thosecomponents that are identical.

In FIG. 4, there is shown a partial sectional view of the tank 24 in thearea of one of the fittings 32. The same reference numerals are used inFIG. 4 as those used in FIG. 2 to describe those components that areidentical to one another. Tank 24 has an opening 40 that issubstantially closed off by cover 42, which is welded to the tank bywelds 44. Similarly, the fittings 32 are welded to the cover 42 by welds45. The cover 42 has a hole (not shown) therein to allow access througha corresponding opening (not shown) in the fitting 32. A bonding agent46 is applied to an inner surface of the cover 42. The thickness of thebonding agent 46 is exaggerated for purposes of illustration.

It can be seen that the inner tank 28 is bonded to the cover 42 in thearea where the bonding agent 46 has been applied (one of the firstareas) and separated from the outer tank 26 by an interstice 30 in theremaining circumference of the tank 24.

In FIG. 5, the cover 42 has been omitted and the fitting 32 is weldeddirectly to the outer tank 26. The bonding agent 46 is coated onto aninner surface 48 of the tank 24 and the inner tank 28 is bonded to theinner wall 48 of the outer tank 26 in the area of the fitting 32, butseparated from the inner wall 48 through the remaining area of the tank24. In FIG. 5, the fitting 32 covers an opening (not shown) in the tankwall 48.

In FIG. 6, there is shown a sectional view of the tank 24 through thefitting 32. It can be seen that the fitting 32 has screw threads 50 inan opening 52 that contains a plug 54. Unthreaded flanged fittings mayalso be used in place of the threaded fittings. An interior surface 56of the plug 54 is coated with bond release material 58. The thickness ofthe bond release material is exaggerated for ease of illustration. Thosecomponents of FIG. 6 that are identical to the components of FIG. 4 aredescribed using the same reference numeral. The opening 52 extendsthrough the cover 42.

During manufacture, the opening 52 of at least one of the fittings 32 isopen and thermoplastic material in powder form is inserted into theouter tank 26 through the opening 52. At the same time, bonding agent 46is coated onto an interior surface of the cover 42 (or, if there is nocover, the bonding agent is coated onto an inner surface 60 of the tankaround the fitting 32). A breather pipe is installed into one opening inthe tank to vent off gases formed during the rotational molding process.All other openings are then closed off by plugs 54 preferably containingmold release material 58 on the inner end 56 thereof. Preferably, theplugs are made from teflon or are teflon coated.

The outer tank 26 is then rotationally molded and heated during therotational molding process to melt the thermoplastic powder and coat theinner surface 48 of the outer tank 26 with thermoplastic material.

As a result of the molding process, after the inner tank 26 and outertank 28 are cooled the inner tank 28 falls away from the outer tank 26in all areas except for those areas that were coated with bonding agent.The bonding agent is not affixed to the tank on the interior surface ofthe plug because of the presence of the bond release material 58. Theplug is then removed from the opening 52 and a hole is cut through thethermoplastic material located directly beneath the opening 52. The tank24 is then ready for use, permitting the attachment of threaded pipingdirectly to the threaded fittings of the tank.

Meanwhile many thermoplastic materials are suitable for the inner tank,the most commonly used materials are polypropylene, nylon (a trade mark)and polyethylene. Usually, the tank openings will be located along a topof the tank, but openings can be located at any convenient location inthe outer tank wall. In FIG. 7, there is shown a portal sectional viewof an outer tank 62 having a flanged fitting 64. Those components ofFIG. 7 that are identical to the components of FIG. 6 are describedusing the same reference numeral. The flange fitting 64 is preferablycoated with bonding agent 46 within the fitting 64 from an outer point66 to an inner point 68 around the flanged fitting 64. This causes theinner part of the flanged fitting 64 to be coated with thermoplasticmaterial 70 that is part of the inner tank 28. It should be noted thatthe bonding agent 46 coats an inner corner 72 of the flanged fitting 64to eliminate the corner 72 as a high stress point when the inner tankcools and separates from the outer tank 62. While shrinkage of the innertank during cooling can be controlled by controlling internal gaspressure within the tank 28, shrinkage cannot be easily controlled inthe area between two flanged fittings. However, by bonding the innertank to the outer tank within the flanged fittings and beyond the corner72 the risk of the inner tank failing prematurely because of stresscaused by shrinkage is greatly reduced.

1. A double wall tank comprising an outer tank of steel and an innertank of thermoplastic material, said outer tank having an inner surfacecomprising first areas and a second area, said first areas being locatedadjacent to each of at least two fittings that are separate and apartfrom one another, said second area being a remainder of said innersurface that is not said first areas, the thermoplastic material beingbonded to said outer tank in said first areas and being separate fromsaid outer tank in said second area.
 2. A tank as claimed in claim 1wherein the thermoplastic material of said inner tank is selected fromthe group of polypropylene, nylon and polyethylene.
 3. A tank as claimedin claim 1 wherein an opening of each of said fittings is closed by acover to which said thermoplastic material is bonded.
 4. A tank asclaimed in claim 1 wherein said fittings are threaded fittings.
 5. Atank as claimed in claim 1 wherein there is a vacuum for monitoring aninterstice between said inner tank and said outer tank for leaks.
 6. Atank as claimed in claim 1 wherein said inner tank is substantially ofuniform thickness.
 7. A tank as claimed in claim 1 wherein the fittingsare located along one side of the tank.
 8. A tank as claimed in claim 1wherein the fittings are not lined with thermoplastic material.
 9. Atank as claimed in claim 1 wherein the tank has flanged fittings andsaid flanged fittings are lined with thermoplastic material.
 10. A tankas claimed in claim 1 wherein the inner tank is self supporting, exceptfor being bonded at the fittings to the outer tank.
 11. A method ofconstructing a double wall tank having an outer tank of steel and aninner tank of thermoplastic material, said outer tank having an innersurface with first areas and a second area, said first areas beinglocated adjacent to each of at least two fittings that are separate andapart from one another, said second area being a remainder of said innersurface is that is not said first areas, said method comprising applyingbonding material to said first areas, inserting thermoplastic materialin powder form into said outer tank through an opening for a fitting,said powder including mold release material, subjecting the outer tankto heat and rotating said outer tank to mold said inner tank within saidouter tank, after cooling said tanks, having an inner wall of saidthermoplastic material that is bonded to said first areas and separatedfrom said second area, removing each plug and cutting an opening throughsaid thermoplastic material at each opening.
 12. A method as claimed inclaim 11 including the step of controlling shrinkage of said inner tankduring coding by controlling pressure of gas within said tank.
 13. Amethod as claimed in claim 11 including the steps of covering openingsfor said fittings with covers, applying bonding material to an interiorsurface of said covers, cutting openings into said covers after saidrotational molding process has been completed and installing fittings.14. A method as claimed in claim 12 including the steps of coveringopenings for said fittings with covers, applying bonding material to aninterior surface of said covers, cutting openings into said covers aftersaid rotational molding process has been completed and installingfittings.
 15. A method as claimed in claim 11 including the step ofinstalling threaded fittings.
 16. A method as claimed in claim 12including the step of installing threaded fittings.